1. Field of the Invention
This invention relates generally to detecting high impedance faults and more particularly to system and method of identifying false positive indications of high impedance faults in electric power transmission and distribution networks.
2. Description of the Related Art
Three phase distribution networks carry energy along three separate conductors while maintaining a line voltage difference between any two of the conductors. Under normal operating conditions, the three voltages are symmetrical about a neutral point. Voltage measured between a conductor and the neutral point is referred to as the phase voltage.
Where star (Y) connected transformer windings are used at a source station, the star point corresponds to the neutral position. For certain circuits, this neutral point is brought out as a fourth conductor and can carry load current for loads connected between a phase and a local ground or earth. This is the common arrangement for 120 volt low voltage (LV) circuits and is used extensively in the United States.
Each conductor or phase also has a voltage with respect to its surrounding environment, mainly the local ground or earth. Load current normally flows out from one or more of the phases and returns through the other phase wire(s). In the event of a fault such as a conductor coming in contact with the physical ground, some current flows into the ground. This fault current must find its way back to the source, i.e., into the electrical network. The network neutral point is generally connected to earth at the source to provide such a return path. Fault current for a network earth fault generally flows from the phase conductor, through earth to the neutral-earth connection.
There are many ways to connect a network neutral to earth. In one example, the neutral point is left isolated. In this case, fault current flows back into the system through a weak capacitive connection between earth and the remaining phase wires. Relatively little fault current can flow through this capacitive link in the event of a fault, and faults are difficult to detect. However, by taking certain precautions, these faults can be tolerated on a network because it is not necessary to immediately trip the circuit and interrupt a supply to customers. Isolated neutral circuits are predominantly used on rural overhead networks, where faults are frequent, damage is less severe, and it is desirable to minimize frequent outages.
In another example where the neutral is connected to earth, impedance such as a resistor is used in the neutral-earth link to limit the amount of current that can flow during a fault. When low impedance devices are used, faults can be detected easily but must be cleared quickly. This arrangement is referred to as a low impedance earth network. When high impedance devices are used, fault currents are limited so that damage is limited and faults can be sustained on the network. This arrangement is referred to as a high impedance earth network. Isolated neutral networks are classified as high impedance earth networks because the capacitive coupling is effectively a high impedance link to earth.
High impedance earth networks offer better operational performance in rural overhead networks when earth faults can be detected reliably. Once a fault is detected, the simplest corrective measure is to trip the circuit. However, this can be very disruptive to customers when frequent fault events occur. Where it is desired to maintain a supply of power in the event of a fault, the fault current must be brought to a very low level at the fault site.
There are two method used for reducing fault current at a fault site. One method is to install an arc-suppression coil in the neutral-earth link at the source station. The coil diverts or tunes out most of the fault current. A second method involves the use of an earth switch to connect the faulted phase directly to earth at the source station. This switching shorts out the fault by diverting the bulk of the fault current directly to the station. This method is referred to as a faulted phase earth connection.
The impedance of the fault at the fault site also determines the amount of current that flows in an earth fault. Low impedance faults facilitate current flow, allowing the fault to be readily detected, whereas high impedance faults restrict current flow, making detection more difficult.
A typical single phase high impedance fault occurs when, for example, a tree branch contacts a high voltage distribution power line. This often results in an arc between the tree and the power line, which is a high impedance fault. High impedance faults are also associated with fallen conductors and are extremely dangerous despite a restricted current flow.
There are factors that mitigate against high impedance fault detection. During the normal operation of a network, there is always a certain amount of current flowing to earth through the capacitive links between each phase conductor and earth. When one of the phase wires is switched, for example, during normal operations or load switching, these currents can be interrupted or become unbalanced. Unbalance currents flow in the earth and in any neutral-earth link, and can appear as an earth fault. As these are normal operational events, a protection system must discriminate between such events and real faults.
These issues arise in relation to overhead medium voltage (MV) distribution networks, which are much more extensive than underground networks, because the conductors are exposed to weathering. Additionally, bare conductors located above ground increase the risk of exposing the public to hazards in the event of an accident.
In a high impedance earth network, the phase voltages during an earth fault are disturbed. These disturbances can be used to detect earth faults in a protection system that is also affected by operational events and must operate with limited sensitivity. To limit spurious operation, a threshold of a maximum possible fault current is set, for example, at a 15 amp fault current in a 10 kV network. Therefore, any fault below 5 A will not be detected.
In a low impedance neutral system, when an earth fault occurs, the faulted phase must be tripped, which has continuity and supply quality implications. With high impedance earth circuits, earth faults may be sustained, which can, in turn, have safety implications. High impedance earth circuits impose higher voltage stress on a network and can lead to faults which can have more serious consequences than a simple earth fault.
The type of neutral earth circuit used determines the fault currents and voltages that occur during an earth fault. With a low impedance neutral earth circuit on an existing 20 kV network, conventional over-current type protection may be used to provide a reasonable degree of protection. However, where high impedance neutral earth detection is required, a more sophisticated technique is needed.
If high impedance fault protection of a neutral earth circuit of a rural MV network is required, two options are available. First, arc-suppression with a coil is frequently used. However, there are significant difficulties with arc-suppression in rural networks. The widespread use of single phase spurs in a rural network and the consequent phase imbalance can cause neutral voltages that are above earth potential and a continuous voltage stress on the network.
A very dangerous type of fault is a fallen conductor, which brings high voltage down to ground level and within reach of the public. Where the fallen conductor is on the source side of a break, that is, connected directly to the source, there is a reasonable probability of fault detection depending on ground conditions. Where the fallen conductor is on the load side of a break, the fault can be considerably more difficult to detect because the fault current must first flow through the downstream load before reaching the fault. If there is a small load at the time of the break, such as for example at night, the load impedance may be very high.
For example, a 40 kV A single phase load at 20 kV would correspond to a load impedance of about 12,000 ohms. This will limit the earth fault current to about 1 amp. The ground potentials at the fault site may be low but somebody touching the conductor could be exposed to a lethal voltage. It is, therefore, desirable to detect as many of these faults as is possible.
The use of extensive single phase networks for rural MV networks can result in significant imbalances. Furthermore, imbalances that are caused by single pole switching and fuse blowing are very similar to earth faults. For example, opening a link or blowing a fuse on a 50 km long single phase spur can result in 50 km of a conductor being removed from one phase and added to another phase through the load. The effect is to cause an unbalanced current of less than about 2 amps in the neutral conductor of a 20 kV network. Similar events in a three phase network have even greater effects, especially when the switching occurs close to the source station.
The use of boosters in an open delta configuration introduces an additional complication. When a network is parallel-connected through a booster, the open delta configuration causes a shift in the position of the neutrals between the two networks. This neutral shift causes a circulating current in the neutrals of an earth neutral 20 kV network or neutral displacement in the case of a 10 kV network. A high impedance fault detection system should distinguish between such effects and a real earth fault.
Single pole switching to parallel networks introduces a voltage displacement between the neutral points. If there is a difference in the voltage drops between the networks where they are parallel-connected, the difference is taken up in the neutral points. This difference causes circulating currents in the neutrals, which from the source, appear very similar to an earth fault. Again, adequate earth fault protection should be able to discriminate between a switching event and an earth fault. The demands on sensitive earth fault protection systems for rural networks are particularly severe due to the presence of single phase spurs.
There have been several efforts to use the transient effects arising from a fault to detect high impedance faults. However, different solutions have been developed for differing classes of faults and no universal solution based on analyzing such transient effects has evolved. Present systems attempt to amalgamate the existing various techniques into a composite system to apply the most appropriate technique to a particular fault. Alternatively, all the available techniques are applied simultaneously and a consensus is derived between them on whether there is a fault or not.
However, of all the various systems and/or methods for detecting high impedance faults, none are directed to distinguishing between a false positive indication of a high impedance fault and an actual high impedance fault.
U.S. Pat. No. 5,659,453 discloses an apparatus for detecting arcing faults on power lines carrying a load current by identifying bursts of each half cycle of the fundamental current. While the apparatus and method shown is highly suited to the detection of arc faults, it does not show or suggest a method for detecting permanent, non arcing faults or high impedance faults.
U.S. Pat. No. 4,871,971 uses phase shifts to detect faults. In this reference, it is noted that phase shifts are not typical for most occurrences of high impedance faults because most of the occurrences of high impedances faults often incorporate resistant type effects. Usually, with high impedance faults, no inductance/capacitance is involved, and these are the two effects that lead to phase shifts, while resistance type anomalies manifest themselves rather in amplitude disturbances.